The invention relates to a box spanner with a device for retaining the object to be turned.
Box spanners, in particular for turning sparking plugs of engines for motor vehicles, which have means for gripping the object to be turned are known. These gripping means make it possible to avoid pushing the plug, or the fastening component to be connected, onto the threaded seat, such as for example the threaded seat of the cylinder head of the engine, in advance, before using the box spanner for tightening, and make it possible to withdraw the component easily when unscrewing it, in particular in cases of difficult access or close to sources of heat, such as the cylinder head of an engine.
With these tools, it is also possible to work in very confined and deep spaces with the aid, for example, of auxiliary turning tools and, if necessary, of extensions, rendering the application of the desired twisting torque to the object to be screwed in or unscrewed both accurate and easy.
Currently, an embodiment of the retaining device of the box spanner for spark plugs consists of a ring of elastic material located in the space of the box spanner above the polygonal seat for transmission of the twisting torque. This ring, fixed to the walls of the hollow space of the spanner, has fingers projecting towards the centre of the space, which leave a central opening of a size which is smaller than the transverse dimension of the ceramic extension of the plug.
During the operation of screwing or unscrewing the plug, the ceramic body, which extends beyond the tightening nut of the plug, fits into the hollow space of the box spanner, engaging with an interference fit between the fingers projecting from the elastic ring until the polygonal surface of the nut of the plug mates with the likewise polygonal seat formed inside the mouth of the box spanner.
The disadvantage of this embodiment lies in the limited temperature variation which the elastic ring can tolerate in use. This is an unacceptable limitation of the use of the box spanner as it is essential to work both at low climatic temperatures and at the high temperatures found on internal combustion engines directly after operation. With this embodiment, however, at low temperatures, the elastic ring, which is usually made of rubber, loses elasticity, which results in a loss of grip and slipping of the plug.
On the other hand, at high temperatures, there is a tendency for the elastic ring to loosen with respect to the spanner, which brings about slipping of the ring.
A second embodiment of the retaining device for a box spanner, which is known from the prior art, consists of a permanent magnet of hollow cylindrical shape fixed to the internal surface of the hollow space of the box spanner above the polygonal seat for turning the plug. The permanent magnet of cylindrical shape has a coaxial hole of a size which is necessarily smaller than the cavity of the box spanner but large enough for the passage of the ceramic extension of the plug.
Retention of the plug is provided by the action of the permanent magnet on the metal part of the plug, which is located at the bottom of the ceramic extension.
This embodiment has the disadvantage that the cylinder of magnetic material forms a projection which partially obstructs the hollow space of the box spanner. Insertion of the plug in a manner which is not perfectly coaxial with the box spanner, which is highly likely if the spanner is being used in a situation with difficult direct access to the threaded member, brings about scraping of the very delicate surface of the ceramic extension of the plug by the magnet. This scraping may lead to damage of the ceramic part which, even if only slight, causes the plug not to function.
Breakage of the plug, which sometimes cannot even be seen, is very serious, in particular in engines with catalytic devices, since non-functioning of the spark plug may compromise the integrity of the catalytic converter resulting in very costly damage.